Accurate control of qubits is the central requirement for building functional quantum processors. For the current superconducting quantum processor, high-fidelity control of qubits is mainly based on independently calibrated microwave pulses, which could differ from each other in frequencies, amplitudes, and phases. With this control strategy, the needed physical resource could be challenging, especially when scaling up to large-scale quantum processors is considered. Inspired by Kane’s proposal for spin-based quantum computing, here, we explore theoretically the possibility of baseband flux control of superconducting qubits with only shared and always-on microwave drives. In our strategy, qubits are by default far detuned from the drive during system idle periods, qubit readout and baseband flux-controlled two-qubit gates can thus be realized with minimal impacts from the always-on drive. By contrast, during working periods, qubits are tuned on resonance with the drive and single-qubit gates can be realized. Therefore, universal qubit control can be achieved with only baseband flux pulses and always-on shared microwave drives. We apply this strategy to the qubit architecture where tunable qubits are coupled via a tunable coupler, and the analysis shows that high-fidelity qubit control is possible. Besides, the baseband control strategy needs fewer physical resources, such as control electronics and cooling power in cryogenic systems, than that of microwave control. Of note, the flexibility of baseband flux control could be employed for addressing the nonuniformity issue of superconducting qubits, potentially allowing the realization of multiplexing and cross-bar technologies and thus controlling large numbers of qubits with fewer control lines. We thus expect that baseband control with shared microwave drives can help build large-scale superconducting quantum processors.

Exciton polaritons in metallic nanocavities and transition-metal dichalcogenide monolayers has led to striking discoveries, ranging from Bose-Einstein condensation to slowing light. Although plasmonic nanocavities offer small mode volumes, the intrinsic losses of plasmonic nanocavities remain an open challenge in exciton-polariton devices. Consequently, dielectric nanocavities are used as an alternative candidate due to their low intrinsic losses. However, large mode volumes are a central bottleneck in dielectric nanocavities. Here, we theoretically propose to use a hybrid dielectric-metallic nanocavity to enhance light-matter interactions between the excitons of the ${\mathrm{WS}}_2$ monolayer and the hybrid nanocavity. Such a hybrid nanoresonator inherits the advantages of both dielectric and metallic nanocavities, including ultrasmall mode volume, ultralow losses, and reasonably-high-Q factor. It is demonstrated that the thickness and material of the central gap film together with the thickness of the metallic substrate play vital roles in governing the coupling strength between 1L-${\mathrm{WS}}_2$ excitons and the cavity. After optimizing the geometry and material parameters, the Rabi splitting is increased to 113 meV, almost twice that in dielectric metasystems. The significant improvement can be attributed to the greatly enhanced near field and the ultrasmall mode volume. Furthermore, we show that Rabi splitting can be further boosted to 151 meV by increasing the number of layers of ${\mathrm{WS}}_2$ and h-$\mathrm{BN}$ film in the nanocavity.

Quantum entanglement has become an essential resource in quantum information processing. Existing works employ entangled quantum states to perform various tasks, while little attention is paid to the control of the resource. In this work, we propose a simple protocol to upgrade an entanglement source with access control through phase randomization at the optical pump. The enhanced source can effectively control all users in utilizing the entanglement resource to implement quantum cryptography. In addition, we show this control can act as a practical countermeasure against memory attack on device-independent quantum key distribution at a negligible cost. To demonstrate the feasibility of our protocol, we implement an experimental setup using just off-the-shelf components and characterize its performance accordingly.

${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_2$ has recently attracted significant interest as a promising candidate for applications in green light-emitting diodes and photovoltaic absorbers. However, the experimentally synthesized ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_2$ samples suffer from a high concentration of unintentionally doped electrons, and their origin is not fully understood yet. By performing first-principles calculations, we investigate the properties of intrinsic point defects and oxygen impurities in cation-ordered and -disordered wurtzite ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_2$. It is found that the cation antisite defect ${\mathrm{Sn}}_{\mathrm{Mg}}$ is the predominant donor defect, contributing an electron concentration as high as ${10}^{17}{\mathrm{cm}}^{-3}$ to cation-ordered ${\mathrm{Mg}\mathrm{Sn}\mathrm{N}}_2$. However, this value is 2–3 orders of magnitude lower than experimental observations (${10}^{19}-{10}^{20}{\mathrm{cm}}^{-3}$). We further show that cation disorder significantly decreases the formation energies of defects, leading to a much higher electron concentration (${10}^{19}{\mathrm{cm}}^{-3}$) than in the case of the cation-ordered phase. The reduced formation energy can be understood by the energy gain due to the creation of energetically favorable local motif structures after the formation of defects. Hence, reducing the degree of cation disorder is beneficial for decreasing the densities of defects and self-doped carriers.

Inspired by the neurosynaptic frameworks in the human brain, neuromorphic computing is expected to overcome the bottleneck of traditional von Neumann architecture and be used in artificial intelligence. Here, we predict a class of potential candidate materials, $M{\mathrm{Cu}}_3{X}_4$ ($M$ = $\mathrm{V}$, $\mathrm{Nb}$, $\mathrm{Ta};X=\mathrm{S},\mathrm{Se},\mathrm{Te}$), for neuromorphic computing applications through first-principles calculations based on density-functional theory. We find that when $M{\mathrm{Cu}}_3{X}_4$ are inserted with $\mathrm{Li}$ atom, the systems would transform from semiconductors to metals due to the considerable electron filling [approximately 0.8 electrons per formula unit (f.u.)] and still maintain well-structural stability. Meanwhile, the inserted $\mathrm{Li}$ atom also has a low diffusion barrier (approximately 0.6 eV/f.u.), which ensures the feasibility to control the insertion and extraction of $\mathrm{Li}$ by gate voltage. These results establish that the system can achieve the reversible switching between two stable memory states, i.e., high or low resistance state, indicating that it could potentially be used to design synaptic transistor to enable neuromorphic computing. Our work provides inspiration for advancing the search of candidate materials related to neuromorphic computing from the perspective of theoretical calculations.

Space-time-varying metamaterials based on uniform-velocity modulation have spurred considerable interest over the past decade. We present here an extensive investigation of accelerated-modulation space-time metamaterials. Using the tools of general relativity, we establish their electrodynamic principles and describe their fundamental phenomena, in comparison with the physics of moving-matter media. We show that an electromagnetic beam propagating in an accelerated-modulation metamaterial is bent in its course, which reveals that such a medium curves space-time for light, similarly to gravitation. Finally, we illustrate the vast potential diversity of accelerated-modulation metamaterial by demonstrating related Schwarzschild holes.

${\mathrm{Cr}}^{2+}$ - and ${\mathrm{Fe}}^{2+}$ -doped $\mathrm{Zn}\mathrm{Se}$ crystals are laser materials with phonon-broadened absorption and emission spectra, which provide broadband laser gain in the 2- to 5-$\text{μ}\mathrm{m}$ wavelength range. While ${\mathrm{Cr}}^{2+}\text{:ZnSe}$ can be directly pumped with high-power Er, Ho, or Tm lasers, no such possibility exists for ${\mathrm{Fe}}^{2+}\text{:ZnSe}$. To this end, electronic excitation transfer between ${\mathrm{Cr}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ ions in codoped ZnSe is investigated as an alternative excitation process in photo luminescence (PL) experiments with sub-10-ns temporal resolution. For a wide range of ion concentrations, we observe nonexponential decays of ${\mathrm{Cr}}^{2+}$ PL on a microsecond time scale, a 60-ns rise time of ${\mathrm{Fe}}^{2+}$ PL at high doping densities, and a prolonged decay of ${\mathrm{Fe}}^{2+}$ PL due to the temporal characteristics of excitation transfer over a range of interionic distances. Using a multirate equation model, the transfer process is analyzed on length scales up to 30 nm and compared to the established continuum model approach. The analysis reveals an unexpectedly efficient excitation transfer from ${\mathrm{Cr}}^{2+}$ to ${\mathrm{Fe}}^{2+}$ ions with an enhancement of the excitation transfer rates by up to a factor of 5 in comparison to resonant dipole-dipole coupling. The enhancement is assigned to (multi)phonon-assisted excitation transfer, in analogy to the phonon-mediated efficient radiationless decay of the excited ${\mathrm{Fe}}^{2+}$ state. As nonradiative losses and excitation transfer show different temperature scaling, a cryogenic temperature regime is found that promises overall efficiencies above 50%, making ${\mathrm{Fe}}^{2+}:{\mathrm{Cr}}^{2+}\text{:ZnSe}$ a much more viable alternative to parametric conversion schemes in the midinfrared range.

Dark-field imaging is an x-ray technique used to highlight subpixel, typically micrometer-scale, density fluctuations. It is often used alongside standard attenuation-based and also phase-contrast x-ray imaging, which both see regular use in tomography. We present x-ray dark-field computed tomography (CT) with a laboratory edge-illumination setup. The dark-field contrast is shown to increase linearly with the x-ray path length through the imaged object, a prerequisite for the use of standard tomographic reconstruction approaches. A multimaterial, custom-built phantom is used to show how dark-field contrast CT can complement attenuation contrast CT for the separation of materials based on their microstructure. As an example of a more complex, biological sample, we present a model rat heart. We show, by comparison with attenuation contrast tomography, that dark-field enables the identification of additional structures undetected through the attenuation contrast channel, as well as offering a consistently sharper reconstructed image.

Electromagnetic wave interaction with time-varying systems has gained a lot of research interest in recent years. The temporal modulation gives unprecedented control over the response, allowing us to go beyond the state of the art in passive systems. In this work, we use time variation and show it can enhance the direction-of-arrival sensitivity of a deeply subwavelength dimer, using the added to degrees of freedom. We formulate the problem, derive an analytical model, and discuss the various physical mechanisms responsible for the enhancement. We show that time modulation enables an alternative degree of control that can be used to optimize the response for various incident frequencies, allowing for wideband operation. Additionally, we show that incorporating the currents from higher generated harmonics into the sensing scheme allows us to extract more accurate information about the impinging wave.

We present a single-beam atomic magnetometer operating at room temperature for the measurement of ac magnetic fields. The magnetometer functions in the nonlinear regime of magneto-optical rotation of ${}^{85}\mathrm{Rb}$ atomic vapor without any buffer gases. We demonstrate a sensitivity of approximately $0.9\mathrm{pT}/\sqrt{\mathrm{Hz}}$ at 2 kHz and a large bandwidth of 24 kHz. The dynamic range of measurement is ${10}^6$, making the sensor effective even in Earth’s field. We present the signal-to-noise and bandwidth characteristics of the system for both shielded and unshielded modes of operation. Moreover, we perform theoretical analysis for the atom-light system for the single-laser-beam configuration. The effect of light intensity and detuning on the magnetometer is studied theoretically as well as experimentally to understand the strengths and limitations of the technique.

Application of various electron-transport layers (ETLs) with desired morphology and dimensions is a key factor to be considered for the structural designs of perovskite solar cells (PSCs) in order to obtain enhanced optical and electrical properties. The metal-oxide ETLs composed of one-dimensional nanorod arrays are one of the most frequently used nanostructures for electron transporting in PSCs. In this work, computer-simulation methods are employed to investigate the optoelectrical properties of PSCs with planar and nanorod-based ${\mathrm{Sn}\mathrm{O}}_2$ ETLs. A two-dimensional optical model is used to determine the optical responses of devices and the standard drift-diffusion model is utilized to simulate their behavior and performance. The aspect ratio and the density of ${\mathrm{Sn}\mathrm{O}}_2$ nanorods are varied to examine their effect on the optical and the electrical properties of resulting devices and the findings are contrasted with those of planar devices. It is found that under optimum conditions, PSCs with thin planar ${\mathrm{Sn}\mathrm{O}}_2$ ETLs, which are referred to as reference planar devices, outperform PSCs with nanorod-based ${\mathrm{Sn}\mathrm{O}}_2$ ETLs even if the light-harvesting properties of the latter are improved with the implementation of structured ETLs. The underlying reasons for this phenomenon are analyzed and rationalized through a detailed analysis and comparison of electric field, current density, and carrier recombination distributions in PSCs. The findings of this work can be used as a theoretical guide for designing and fabrication of high-performance planar and structured PSCs using ${\mathrm{Sn}\mathrm{O}}_2$ ETLs.

Different from the traditional bulk-edge correspondence principle, the discovery of higher-order topological states has generated widespread interest. In a second-order, two-dimensional elastic wave topological insulator, the fluctuation information can be confined to the corners, with the state being topologically protected. To better apply the topological corner states, a cleverly designed mechanical structure is used to achieve the adjustment of the topological corner states on a two-dimensional elastic plate. The energy-band inversion can be achieved by switching the material of the inner and outer brackets of the unit cell of the honeycomb elastic plate. The topological corner states can be observed at the boundary of two different topological phase structures in the finite lattice using finite-element software. The robustness of the topological corner state is verified by setting up defective control groups at the corner. In addition, the topological corner states of this honeycomb elastic plate are discussed in terms of positional tunability. The honeycomb elastic plate can provide a reference for the design of devices for the local control of elastic waves and energy harvesting due to the possibility of modulation of topological corner states, which is beneficial for the application of topological corner states in practice.

Projective quantum measurement is fundamental for quantum technologies. Here we propose a method for construction of projective measurement of nonconserved quantity of nuclear spin via a weakly coupled auxiliary electron spin. The key idea is to apply suitable quantum control on the electron to construct a weak quantum nondemolition (QND) measurement on the nuclear spin and then cascade a sequence of such measurements into a projective one. We identify a set of tunable parameters to select the QND observables and control the strength of the weak QND measurement. We also find that the QND measurement can be stabilized against realistic experimental control errors. As a demonstration of our method, we design projective measurement on a ${}^{13}\mathrm{C}$ nuclear spin weakly coupled to a nitrogen-vacancy center electron spin in diamond.

A steerable parametric loudspeaker array is known for its directivity and narrow beam width. However, it often suffers from the grating lobes due to periodic array distributions. Here we propose the array configuration of hyperuniform disorder, which is short-range random while correlated at large scales, as a promising alternative distribution of acoustic antennas in phased arrays. Angle-resolved measurements reveal that the proposed array suppresses grating lobes and maintains a minimal radiation region in the vicinity of the main lobe for the primary frequency waves. These distinctive emission features benefit the secondary frequency wave in canceling the grating lobes regardless of the frequencies of the primary waves. Besides, the hyperuniform disordered array is duplicatable, which facilitates extra-large array design without any additional computational efforts.

Heralded single-photon sources are a fundamental building block for optical quantum technologies. These sources need to be unfiltered and integrated to have good scalability and avoid unnecessary losses. To attain this goal, it is necessary to control the effective nonlinearity seen by the fields as they mix and propagate in a waveguide source. In this paper, we introduce a method to design nonlinear waveguides with arbitrarily shaped effective nonlinearity profiles. The method takes advantage of the fact that the second-order nonlinear response is a tensor quantity and thus the local effective nonlinearity of a material depends on the propagation direction of the fields participating in the interaction. Thus, by locally changing the propagation direction of the fields we can modulate the wave-mixing process. Our method allows for the waveguide fabrication process to be significantly simplified. The material structure of the waveguide is made by a single crystal, no longer needing oriented patterning or periodic poling. We use our method to design waveguides with a nonlinearity profile that is Gaussian in the propagation length, allowing the generation of perfectly pure heralded single photons.

Defect-free atomic arrays have been demonstrated as a scalable and fully controllable platform for quantum simulations and quantum computations. To push the qubit size limit of this platform further, we design an integrated measurement and feedback system, based on field-programmable gate array (FPGA), to quickly assemble two-dimensional defect-free atomic array using maximum parallelisms. The total time cost of the rearrangement is first reduced by processing atom detection, atomic occupation analysis, rearrangement strategy formulation, and acousto-optic deflectors driving signal generation in parallel in time. Then, by simultaneously moving multiple atoms in the same row (column), we save rearrangement time by parallelism in space. To best utilize these parallelisms, we propose an alternative algorithm named the Tetris algorithm to reassemble atoms to arbitrary target array geometry from two-dimensional stochastically loaded atomic arrays. For an $L\times L$ target array geometry, the number of moves scales as $L$, and the total rearrangement time scales at most as $L^2$. Although in this work we do not test on actual atoms, we simulate the performance of our FPGA system experimentally with all components integrated except for the atoms. We present the overall performance for different target geometries, and demonstrate a dramatic boost in rearrangement time cost and the potential to scale up defect-free atomic array to 1000 atoms in room-temperature platform and 10 000 atoms in cryogenic environment.

Identifying clusters in data is an important task in many fields. In this paper, we consider situations in which data live in a physical world, so we first have to collect the images using sensors before clustering them. Using sensors enhanced by quantum entanglement, we can image surfaces more accurately than using purely classical strategies. However, it is not immediately obvious whether the advantage we gain is robust enough to survive data-processing steps such as clustering. It has previously been found that using quantum-enhanced sensors for imaging and pattern recognition can give an advantage for supervised learning tasks, and here we demonstrate that this advantage also holds for an unsupervised learning task, namely clustering.